Detecting the presence of specific chemical compounds in the atmosphere is important in a variety of different applications. For example, it is often important to detect the presence and concentration of potentially flammable compounds in the atmosphere. A chemical compound of interest is often referred to as a target analyte.
A variety of different sensor systems known in the art can be used to detect the presence of one or more of the analytes. Most sensor systems employ a sensing component that absorbs the analytes. The sensing component undergoes physical changes upon absorbing one or more of the analytes.
A sensor system typically comprises a sensor probe that includes both the sensing component and a probe body housing (including terminals for transmitting an output). The terminals are typically coupled to a control unit, also part of the sensor system, which analyzes outputs received from the sensor probe. The control unit is coupled to a user interface, typically including an indicating device, which signals when concentration of an analyte exceeds threshold values.
Many sensor systems employ a sensing component that includes a sensor film. The sensor film absorbs the target analytes when they are present. Upon absorption of the analytes, the sensor film undergoes physical changes. Various sensor systems available in the art measure the physical changes in the sensor film to identify the presence of the target analytes.
Such sensor systems may include optical sensor systems, such as fiber optic sensor systems. In fiber optic sensor systems, a beam of light is projected through an optical fiber at a sensor film cladding. Physical changes (e.g. refractive index or color) in the film are monitored. Changes in refractive index occur when analytes are absorbed and change the physical properties of the cladding (including volumetric changes).
Other sensor systems include surface acoustic wave sensor systems (SAWS). In SAWS systems ultrasonic waves are projected through the sensor film between transducers, which detect any modifications in the properties of the sensor film (primarily the mass), translating those changes into the concentration of analyte present.
Another type of sensor system is a conductiometric sensor system, more particularly, a polymer-absorption chemiresistor sensor system. A polymer-absorption chemiresistor sensor system has a polymer sensor film exposed to a surrounding atmosphere. An electrical charge is applied across the polymer film. The polymer absorbs any target analytes that might be present in the atmosphere. Upon absorption of the target analytes, the sensor film undergoes a volumetric change. This change in volume changes the electrical resistance of the sensor film. Conductive particles may be distributed throughout the polymer film to enhance the sensitivity to resistance changes in the material when the volume of the polymer changes.
Each of the above sensor systems typically include a processor or control unit. The processor monitors the physical properties of the sensor film to determine the absence, presence, and concentration of the target analytes. The processor can be coupled to a user interface. The user interface typically includes an indicating device, which generates a signal when the concentration of the target analyte exceeds a predetermined threshold value.
In chemiresistor sensor systems, the resistance of the sensor film changes not only upon the absorption of the analytes, but also in response to changes in ambient temperature. Because detection of target analytes in chemiresistor sensor systems is based on changes in the resistance of the sensor film that occur when the film absorbs target analytes, changes in ambient temperature that change the resistance of the sensor film can negatively affect the sensor system's ability to accurately detect the presence of target analytes. For example, if the sensor film has a positive temperature coefficient of resistance such that the film increases in resistance upon the absorption of target analytes, increases in ambient temperature might cause the sensor system to generate a false signal indicating that target analytes are present when they are not.
While conventional sensor systems perform adequately for their intended uses, they are subject to improvement. Specifically, it would be beneficial to provide a sensor system that: consumes less power as compared to chemiresistor sensors; has an improved sensitivity/response time; and can reliably identify the presence of target analytes regardless of changes in ambient temperature.